Band-gap reference bias circuits have long been used to produce reference voltages. Practical implementations of band-gap reference circuits take many forms and some embodiments thereof are described in detail in Paul R. Gray and Robert G. Meyer, Analysis and Design of Analog Integrated Circuits, ch. 4, pp. 289-300 (2 ed., 1984). One such band-gap reference circuit 10 is illustrated in FIG. 1. Band-gap reference circuit 10 uses a feedback loop 12 to establish an operating point for the circuit such that the output voltage V out is equal to a base-emitter voltage plus a voltage proportional to the difference between two base-emitter voltages for transistors Q1 and Q2. Band-gap reference circuit 10 is bi-stable as a result of the positive feedback path from the output of operational amplifier A through resistor R1, to the positive input of op-amp A. One stable state is the desired one, with an output V out approximately equal to 1.25 V. In this state, the feedback signal to the negative input of op-amp A dominates. The positive feedback in this state is minimal because of the low impedance of Q1. With zero output voltage (the other stable state), however, Q1 is now in a high impedance state and the positive feedback path will keep V out at 0 V. Accordingly, appropriate start-up circuitry must be included with the band-gap reference circuit 10 to ensure operation in the desired state.
Such a start-up circuit 20 is illustrated in FIG. 2. Start-up circuit 20 is coupled to an input of band-gap reference circuit 10 at node 22. It will be appreciated that if the output voltage V out is zero volts, as may occur at start-up, start-up circuit 20, and in particular, transistor T1, will pull-up the voltage at node 22, causing the output voltage to rise to a desired V out. Feedback loop 12 will then operate to hold the voltage at this desired level.
Unfortunately, start-up circuit 20 is sensitive to process, temperature and power supply variations. If not carefully designed, transistor T1 may still conduct after start-up, i.e., after a desired output voltage V out has been reached. This may interfere with the normal operation of band-gap reference circuit 10, and, as a result, the output voltage V out may fluctuate.